Innate immune recognition of nucleic acid is an important defense mechanism to alert the cell of an invader. Our cells have evolved sophisticated mechanism to distinguish self verse non-self nucleic acid or to contain self nucleic acid to prevent incorrect immune response to healthy cells that often lead to autoimmune diseases. This application seeks to understand the role of TREX1 in the innate immunity and autoimmune diseases. TREX1 is the most abundant 3'-5' exonulease in the cytosol of mammalian cells. Our lab has previously identified TREX1 mutations that are associated with systemic lupus erythematosis (SLE). Two percent of SLE patients harbor mutations in TREX1, representing the single most common cause of monogenic SLE identified. High penetrance causes of lupus are rare and therefore important to study because they have the potential to provide immediate insights into pathogenesis. Other TREX1 mutants have also been identified to associate with Aicardi-Goutieres syndrome (AGS) and retinal vasculopathy and cerebral leukodystrophy (RVCL). Trex1-/- mice develop autoimmune inflammatory phenotypes that are similar to SLE and AGS, such as elevated type I interferon a (IFNa). Cytosolic ssDNA drived from endogenous retroelements during reverse transcription were found to accumulate in Trex1-/- cells that may be the trigger for the IFN response. Using retrovirus to mimic endogenous retroelements, I found that it stimulates the same signaling pathway as described for retroelements in Trex1-/- cells, thereby representing a robust cell-based infection system for studying TREX1 function in the innate immunity. The overall theme of this application is to understand how TREX1 suppresses innate immune response to cytosolic DNA derived from retroelements and retroviruses, and in the absent of TREX1, how DNA generated by reverse transcription (RT DNA) is recognized by the innate immunity. My Aim 1 is to validate innate immune factors uncovered from preliminary studies for their roles in retrovirus-stimulated IFN response. I have identified a few innate immune factors that act positively or negatively on the innate immune response to retrovirus. Validating these factors sets up the framework for studying the underlying pathway. Studies in Aim 2 will define the signaling pathway that recognizes RT DNA and activates IFN using a systematic multidisciplinary approach. Public health relevance TREX1 mutations in humans cause systemic lupus erythematosis (SLE), familial chilblain lupus (FCL), Aicardi-Goutieres syndrome (AGS), and retinal vasculopathy and cerebral leukodystrophy (RVCL). Many of these diseases display autoimmune inflammatory phenotypes such as elevated IFNa. My study will unravel the molecular mechanism of the underlying innate immune signaling pathway. Understanding how TREX1 function in the innate immunity will have broad impact on its genetically linked autoimmune diseases. Innate immune recognition of genetic materials from an invading pathogen alerts the cell by inducing antiviral cytokine responses. Our cell has evolved ways to distinguish self vs non-self nucleic acid such as DNA in the cytosol. Incorrect recognition of self DNA causes immune response to healthy cells that often leads to autoimmune diseases. TREX1 has recently been implicated in inducing interferon in response to endogenous retroelements; and TREX1 gene is mutated in inflammatory diseases of childhood that mimic congenital viral infection and in autoimmune disease, including systemic lupus erythematosis. This study will improve understanding of how TREX1 keeps cytosolic retroelement DNA under the radar of innate immunity, and in the absent of TREX1, how these DNA could be detected. It may also point the way to novel therapies and drug targets for treating autoimmunity.